Homogeneous Photocatalytic Reduction of CO2 to CO Using Iron(0) Porphyrin Catalysts: Mechanism and Intrinsic Limitations

A photochemical catalytic reduction of CO2 was performed in an organic solvent with iron(0) porphyrins as homogeneous molecular catalysts under visible light irradiation. With modified tetraphenylporphyrins consisting of internal phenolic groups, the photochemical process led to the production of CO, with H-2 as a minor product. High catalytic selectivity for CO formation and turnover numbers up to 30 were obtained. Degradation of the catalyst occurred at longer irradiation times, along with decreased selectivity. Furthermore, addition of a weak acid, which increased the reduction efficiency under electrochemical conditions, led to rapid deactivation of the catalyst. With the unmodified tetraphenylporphyrin as catalyst, we observed lower performance and higher proportion of H-2, which highlighted differences in the reduction pathways followed. A combination of a spectroscopic study and product analysis performed under various conditions led to detailed reduction mechanisms and helped pave the way for designing durable photocatalytic systems.